1. Field of the Invention
The present invention relates generally to the field of optical communications. More specifically, the present invention discloses a method and apparatus for wavelength multiplexing and demultiplexing.
2. Background of the Invention
Wavelength division multiplexing is a commonly used technique that allows the transport of multiple optical signals, each at a slightly different wavelength, on an optical fiber. The ability to carry multiple signals on a single fiber allows that fiber to carry a tremendous amount of traffic, including data, voice, and even digital video signals. As an example, the use of wavelength division multiplexing permits a long distance telephone company to carry thousands or even millions of phone conversations on one fiber. By using wavelength division multiplexing it is possible to effectively use the fiber at multiple wavelengths, as opposed to the costly process of installing additional fibers.
In wavelength division multiplexing techniques, multiple wavelengths can be carried within a specified bandwidth. It is advantageous to carry as many wavelengths as possible in that bandwidth. International Telecommunications Union (ITU) Draft Recommendation G.mcs, incorporated herein by reference, proposes a frequency grid which specifies various channel spacings including 100 GHz and 200 GHz. It would be advantageous to obtain 50 GHz spacing. Separating and combining wavelengths with these close spacings requires optical components which have high peak transmission at the specified wavelengths and which can provide good isolation between separated wavelengths.
One technique which has been developed to accomplish the demultiplexing of closely spaced wavelengths is to cascade a series of wavelength division demultiplexing devices, each device having different wavelength separating characteristics. A typical application involves cascading an interferometric device such as an arrayed waveguide device having a narrow spacing of transmission peaks (e.g., 50 GHz) with a second interferometric device which has a coarser spacing and correspondingly broader transmission peaks (e.g., 100 GHz spacing). The cascade of devices provides the separation of wavelengths by subdividing the wavelengths once in the first device, typically into a set of odd and even channels, and then separating wavelengths in the subsets in following devices in the cascade.
Arrayed waveguide (AWG), fused biconical taper (FBT), fiber Bragg grating (FBG), diffraction grating, and other interferometric wavelength demultiplexing devices can be constructed to have the appropriate characteristics for the first or second stage devices in the cascade. However, traditional interferometric devices have the characteristic that as the spacing of the channels is decreased, the transmission peaks become narrower, and are less flat over the wavelength region in the immediate vicinity of each peak than a device with wider channel spacings. As a result, when using a traditional device in the first stage of a cascade, the transmission peaks may not have a high degree of flatness, and any drift or offset of a wavelength from its specified value may result in significant attenuation of that wavelength. In addition, the isolation between wavelengths is frequently unsuitable with conventional interferometric devices and can result in unacceptable crosstalk between channels. With increasing numbers of wavelengths and the close wavelength spacing which is utilized in dense wavelength division multiplexing systems, attenuation and crosstalk must be closely controlled to meet the system requirements and maintain reliable operations. As an example, 40 or 80 wavelengths can be generated using controllable wavelength lasers, with transmission signals modulated onto each laser. It is desirable to be able to multiplex and demultiplex these channels onto one single optical fiber. Although the lasers can be controlled and the wavelengths stabilized to prevent one channel from drifting into another, there is always some wavelength drift which will occur.
In a cascade architecture, the first stage of demultiplexing, or the last stage of multiplexing are where good peak flatness and high isolation are required in order to allow the separation/combining of closely spaced wavelengths.
For the foregoing reasons, there is a need for a wavelength division multiplexing/demultiplexing device which tolerates wavelength drift, maintains a high degree of isolation between channels, and is able to separate/combine large numbers of wavelengths.
The present invention encompasses a method and apparatus for wavelength division multiplexing/demultiplexing in which a cascade is formed between a polarization-based wavelength multiplexing/demultiplexing device and a second wavelength multiplexing/demultiplexing device. The polarization-based wavelength division multiplexing/demultiplexing device has transmission peaks which are substantially flat as a result of the characteristics of a polarization-based filter which is part of the device. In addition, the polarization filter provides a high degree of isolation between adjacent channels. The output of the polarization-based wavelength division multiplexing/demultiplexing device is received by a second stage of wavelength multiplexing devices which further separates the wavelengths. One advantage of the present invention is that the polarization-based wavelength multiplexing/demultiplexing device has good peak flatness and low crosstalk, and permits separation of closely spaced wavelengths (e.g., 50 GHz spacing). The subsequent devices in the cascade can be based on a number of technologies including arrayed waveguide technology, fused biconical taper technology, diffraction grating technology, fiber Bragg grating technology, interference filter, or can also be polarization-based devices. The subsequent devices are used to separate channels which have been formed into subgroups by the polarization-based wavelength multiplexing/demultiplexing device.
In a preferred embodiment the polarization-based wavelength multiplexer/demultiplexer creates two subsets, one subset containing the odd channels from the input channels, the other subset containing the even channels from the input channels. The second device further separates the wavelengths in the first and second subsets, resulting in a wavelength spacing at the output which is 2N times the wavelength spacing at the input of the polarization-based wavelength multiplexer/demultiplexer. In a preferred embodiment the second stage performs the subsequent demultiplexing operations. In an alternate embodiment multiple stages are used in the cascade to further separate the wavelengths and produce a single channel at the final output.
In an alternate embodiment, the polarization-based wavelength multiplexer/demultiplexer separates an upper band from a lower band. Subsequent stages are used to further separate the channels.
When a large number of channels are present, the polarization-based multiplexer/demultiplexer can be utilized to separate the channels into groups, and subsequent stages can be used to continue the multiplexing/demultiplexing process. In a preferred embodiment, when used as a demultiplexer, the present invention separates an input signal into two groups of channels, the even channels and the odd channels. A subsequent stage based on arrayed waveguide (AWG) technology performs the final multiplexing, resulting in individual channels at the output.
In a preferred embodiment the polarization-based wavelength division multiplexing/demultiplexing device is based on a polarization routing device which receives an optical signal carrying multiple channels at various wavelengths, separates the signal into vertical and horizontal polarizations, converts one of the polarizations to be identical to the other polarization, and performs filtering based on the wavelength of the signal, with the polarization of the output being dependent on the wavelength. A polarization routing stage routes light to a particular output depending on its polarization, and a polarization conversion and recombination stage combines the polarizations at each output to form an output signal.
In a preferred embodiment the polarization filter is composed of calcite birefringent crystal plates. A polarization mode dispersion compensator can be utilized in the device to reduce the polarization dispersion.
In the present invention the transmission function obtained in the first stage of a cascade demultiplexer has Fourier components such that the transmission function is substantially flatter and has steeper roll-off from the peaks than the transmission function in the second or subsequent stages. The additional Fourier components in the first stage result in a substantially square transfer function as compared to the second stage transfer function. In a preferred embodiment, the first stage transfer function is realized through the use of a polarization filter, which enables angle to be used as a variable in addition to path length, permitting the incorporation of the additional Fourier components necessary to make a square wave transfer function.
One advantage of the present invention is that it allows the use of low cost interferometric devices in second and higher stages of a wavelength division multiplexing/demultiplexing device while achieving good flatness and low crosstalk through the use of a polarization-based first stage.
In a preferred embodiment a large number of channels (e.g. 40 or 80) with 100 GHz spacing enter the device and are separated according to even and odd channels by a polarization-based first stage device with a spacing of 200 GHz. The second stage device is an arrayed waveguide device which separates the channels into individual channels which appear on the outputs, and which can be individually received by a telecommunications receiving device which is not wavelength selective.
These and other features and objects of the invention will be more fully understood from the following detailed description of the preferred embodiments which should be read in light of the accompanying drawings.